In recent years, a liquid crystal display element is used in various fields as a display element for displaying images. The liquid crystal display element is mainly used for portable electronic devices, and for this type of devices, the major demand is to further reduce power consumption.
Meanwhile, portable electronic devices, especially a mobile phone, are now more often used for transmitting data having large volume of information such as moving image data at high speed, with the development of communication technology. The image refresh frequency (refresh rate; the number of refreshes of a displayed image in unit time) tends to increase in the liquid crystal display element which is used for a mobile phone so as to process such a large volume of information. This tendency is especially remarkable in the case of displaying a moving image. On the other hand, in the case of displaying a still image, it is not necessary to refresh images at a frequency as high as that when displaying a moving image. This is because a display element (a capacitive display element), which is made up of capacitive pixels such as a liquid crystal display element, is capable of holding a displayed state (display image) for a certain period of time once the image is refreshed by externally receiving image data of a pixel unit so that it is not required to refresh images. Note that, the time period of holding the displayed state differs depending on the characteristic of the capacitive display element.
Here, as an example of a conventional display device, the following will explain a display device having a liquid crystal panel 103 as a display element, with reference to the block diagram of FIG. 7.
As shown in FIG. 7, the conventional display device includes a liquid crystal controller 102, and the liquid crystal panel 103 as a display element. The liquid crystal controller 102 includes a display memory 104, a signal synchronization circuit 105, a display timing generation circuit 106, and a data bus 107.
An image data signal S101 which is outputted from an external device such as a CPU is stored in the display memory 104 in the liquid crystal controller 102 via the data bus 107 of the liquid controller 102. The image data signal S101 which is stored in the display memory 104 is read out therefrom as an image data signal S103, and then is sent to the signal synchronization circuit 105.
Meanwhile, a clock signal CK101 is externally inputted to the display timing generation circuit 106 of the liquid crystal controller 102. The display timing generation circuit 106, according to the clock signal CK101, generates a display signal S105 for driving the liquid crystal panel 103, and a synchronization clock signal CK102 for synchronizing the image data signal S103 with the display signal S105. The display timing generation circuit 106 respectively outputs the display signal S105 to the liquid crystal panel 103, and the synchronization clock signal CK102 to the signal synchronization circuit 105. The signal synchronization circuit 105 buffers the image data signal S103 read out from the display memory 104, and outputs the image data signal S103 to the liquid crystal panel 103 as an image signal S104, according to the synchronization clock signal CK102 outputted from the display timing generation circuit 106.
In the foregoing conventional display device, the refresh rate of the liquid crystal panel 103 corresponds to the rate of reading out image data from the display memory 104 and sending the image data to the liquid crystal panel 103 as the image data signal S103, namely, it is the value indicating how many times image signals of one frame (a screen) are sent out from the display memory 104 to the liquid crystal panel 103 in unit time.
Further, in the foregoing conventional display device, the image refresh frequency (refresh intervals) of the liquid crystal panel 103 can be adjusted by varying the frequency of the clock signal CK101 so as to vary the frequency of the display signal S105 which is outputted from the display timing generation circuit 106 of the liquid crystal controller 102 to the liquid crystal panel 103, and the frequency of the image signal S104 which is outputted from the signal synchronization circuit 105 to the liquid crystal panel 103.
However, in the foregoing conventional display device, the image refresh frequency of the liquid crystal panel 103 is constant regardless of whether the image data signal S101 is carrying a moving image or a still image, thereby causing the following problems.
Referring to “normal image refresh intervals” shown in FIG. 8, the image refresh frequency of the liquid crystal panel 103 is commonly set to a relatively high frequency, for example, 60 Hz, which is equal to a frame frequency of a video signal in the NTSC mode, to be suitable for displaying a moving image. In this case, even when a still image is displayed, the image is refreshed in the refresh frequency as high as that when displaying a moving image.
On the other hand, in the liquid crystal panel 103, the displayed images are not required to be refreshed at a high frequency when displaying a still image compared to the case of displaying a moving image. This is because, as already explained, the capacitive display element such as the liquid crystal panel 103 can hold a displayed state (displayed image) for a certain period of time once the image is refreshed, and, when displaying a still image, it is not necessary to refresh the image while the displayed image is held. Therefore, in the foregoing conventional liquid crystal display device, the image refresh frequency for displaying a still image is higher than required.
Thus refreshing images at a frequency higher than required when displaying a still image causes unnecessary power consumption, since the power consumption of the liquid crystal panel 103 increases as the image refresh frequency increases. Namely, in general, when displaying images in the liquid crystal panel 103, power consumption P, which is the power consumed for displaying images in the liquid crystal panel 103, is denoted by the following equation:P=C×F×V2 where C represents a capacitance of the liquid crystal panel 103, F represents a polarity reversing frequency of scanning voltage which is applied to the liquid crystal panel 103, and V represents a voltage applied to the liquid crystal panel 103. The polarity of the scanning voltage is reversed for a specified number of times (the number depends on the mode of driving) in one frame period. Accordingly, the polarity reversing frequency of scanning voltage varies depending on the image refresh frequency for the displayed image. Consequently, the power consumption of the liquid crystal panel 103 increases as the image refresh frequency for the displayed image increases, in other words, as the number of refreshes of the displayed image increases.
As a result, when the image refresh frequency of the liquid crystal panel 103 is set to a frequency corresponding to the frame frequency of a moving image, the liquid crystal panel 103 consumes unnecessary power when displaying a still image, and the power consumption of the display device increases. This poses a problem especially for a battery-powered display device such as a display device used for a mobile phone whose power consumption is expressly required to be reduced.
Further, in order to reduce the power consumption, the number of image refreshes, i.e., the image refresh frequency may be reduced by extending a period of the clock signal CK101. However, referring to “a conventional technique for lowering power consumption” shown in FIG. 8, if the image refresh interval is matched with the frame period of a moving image, a still image is also refreshed at a frequency higher than required. Thus, it is not possible to reduce the power consumption for displaying a still image. Further, the image refresh interval may be reduced to be shorter than the frame period of a moving image; however, it may impair smoothness of the movement of the moving image.
Further, in Japanese Unexamined Patent Publication No. 2000-221923 (published on Aug. 11, 2000), a liquid crystal display device capable of changing the image refresh frequency (refresh rate) based on time information (time reference) which is included in externally inputted image data, is disclosed. Further, a liquid crystal display device capable of reducing the image refresh frequency (extending the refresh interval) in the case where image data does not include a textual image, is also disclosed in this publication.
However, the former invention of the foregoing publication is based on time information included in image data having a particular data structure called MPEG-4 format, and it is not applicable to image data which does not have the time information. This time information is included in compressed image data for indicating a timing to display a frame in MPEG-4 format which performs data compression by thinning out some frame. Thus, the time information is only included in image data of MPEG-4 format and not included in common image data. Accordingly, the former invention of the foregoing publication have a drawback that it is applicable only to the compressed image data of MPEG-4 format, and not applicable to other kinds of image data.
Note that, image data commonly assumed is color signals of R, G and B which are expressed by digital signals, and each pixel has values for R, G and B. For example, in the case of image data of 16 bits, R, G and B are respectively expressed by 5 bits, 6 bits and 5 bits.
Further, in the latter invention of the foregoing publication, in the case where the image data is moving image data which does not include a textual image, or still image data including a textual image, there arises the following problems. Firstly, in this invention, when the moving image data which does not include a textual image is inputted, the image refresh frequency becomes lower than the intended frame frequency (a frame frequency for the inputted moving image data), and smoothness of the movement of the moving image is impaired. Secondly, when the still image data including a textual image is inputted, the image refresh frequency becomes higher than required, thereby causing unnecessary power consumption.